The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
Battery technologies have been gaining enormous importance over the past decade because of the increasing demand for high-power and high-energy density alternative energy systems especially in electric vehicle and portable electronics applications [1]. Various types of batteries have been developed. Among them, lithium-ion (Li-ion) batteries can offer superior electrochemical characteristics including high-energy density, high-voltage, and low self-discharge compared to other types of batteries such as nickel-cadmium and nickel-metal-hydride.5 Because of these unprecedented properties, Li-ion batteries became the most popular battery type in portable electronics such as laptop computers, digital cameras, and cell phones.
Fabrication costs should be considered for the development of practical Li-ion batteries along with making quality electrode and electrolyte materials. A battery cost assessment made by Barnett et al. shows Li-ion batteries produced with the current technology may cost up to $700/kWh7 while DOE is targeting $300/kWh for these batteries by 2015 [3]. Average total cost of the commercial electrolyte, cathode, and anode materials is about $70/kg, while the rest of the cell components cost only $5/kg [7]. For instance, during the production of LiMn2O4 as a cathode material, about 10 MJ/kg energy (approximately 15% of the production energy of a whole battery) is consumed which is the 2nd most costly material in the list [8]. Graphite, which is the current commercial anode material, also has a significant contribution to the fabrication costs (about 8 MJ/kg) [8]. Therefore, new anode materials that will take the graphite's place need to be low-cost and abundant in nature. In addition, each of the components of Li-ion batteries plays important roles in determination of the electrochemical properties. Main important requirements that electrolyte materials should have are high ionic conductivity for Li ions, low electronic conductivity, safety, chemical stability, retention of electrode interface, and limited formation of solid-electrolyte interface (SEI) [9]. Previous studies mostly focused on electrodes instead of electrolyte because the problems in current electrolyte materials such ascarbonates of propylene (PC), ethylene (EC), diethyl (DEC), and dimethyle (DMC) with fairly optimal characteristics seem to be less significant than the problems in electrode materials [10]. The improvement with anode materials also should be done in accordance with the progress on cathode materials in order to make feasible Li-ion batteries. Theoretical specific capacity value of current commercial cathode materials is only a few hundredsmAh/g [11].
Intensive investigation is going on the development of high-capacity cathode materials, yet “high capacity” is theoretically quite limited for intercalation cathodes. High capacity cathode alternatives are limited by the strict chemical criteria for Li-ion batteries [9]. Therefore, it has become more feasible to increase the capacity of anodes rather than that of cathodes [12]. For example, in the case where the specific capacity of a Li-ion cell cathode is 200 mAh/g, the capacity of the anode should be at least 1200 mAh/g in order to reach a theoretical total battery capacity of 86 mAh/g assuming the specific mass (the mass that does not contribute in the capacity) of the other cell components (case, separator, etc.) is 5.8×10−3 g/mAh [12]. Therefore, the limiting factor for the total capacity of a Li-ion battery is considered to be the anode side in the current state-of-the-art cells.
Anode materials made with different modifications of carbonaceous materials have been commercialized and used in Li-ion batteries because of good mechanical stability of carbon. For instance, structural integrity of graphite made from commercial coke (i.e., a carbonaceous material typically obtained from coal) is high enough so that it can tolerate 10% structural change, which is quite small compared to other anode materials. Nonetheless, theoretical maximum capacity limit of graphite has already been reached, and several other materials were tested in order to find new suitable anode materials for Li-ion batteries such as Li-metal and Li-semiconductor composites. Among these materials, significant discoveries have been made in the past decade on the anode materials such as tin (Sn) and silicon (Si). Transition metal oxides also became favorable in recent studies because of their abilities to accommodate more than one Li ion in the unit metal structure, thus making high capacity anodes [14]. However, they have higher lithiation potentials than Si, which limit the cell voltage substantially, and none of them can reach the theoretical capacity value of silicon. The exceptionally impressive properties of Si make it very attractive among others, yet the development of Si anodes is lagged to produce practical batteries because of the problems that will be discussed in the following sections.
Si has become one of the most prominent candidates for lithium-ion battery anodes since it was found to be a reversible host material for lithium intercalation [15]. Structural properties of Li—Si alloys (e.g., Li13Si4, Li22Si5) have been studied and it was shown that although the structure of Li22Si5 was considered to be more favorable for Li-ion battery applications because of its high specific theoretical capacity (about 4200 mAh/g, Table-1), the theoretical value of this morphology or any close capacity values have not been experimentally observed. Instead, capacity of about 3580 mAh/g for Li15Si4 (cubic) structure was observed in recent studies, which is still much better than other candidates [16]. The structure of this phase is assumed to be the same with Cu15Si4 and Li15Ge4 and was able to hold 2.5 Li ions per one Si atom [17].
High Volume Change of Si Anode:
Change of the structural properties of Si during Li insertion/extraction generates a significant problem for Li-ion batteries as in some of the metal and metal-oxide anodes. A vast volume expansion of about 269% in lithiation/delithiation of Li15Si4 causes fracturing and pulverization of the Si anode. This mechanical instability leads to poor charging-discharging cycleability and short battery lifetime. In addition, electronic conductivity of Si, which is already low (about 10−5 S/m), is further reduced due to the cracks and crumbling formed during Li+ incorporation/removal. Therefore, high volume expansion/contraction of Si during lithiation/delithiation reactions eventually becomes the main reason for degradation of its electrochemical properties.
Solid-Electrolyte Interface (SEI) Layer Formation on the Electrode Surface:
SEI layer is a coating incorporating residual materials and Li atoms that forms on the surface of electrodes due to the side reactions occurring during the operation of Li-ion batteries. SEI layer usually emerges in the first few cycles. This initial SEI formation is useful because it acts as a passivation layer and avoids the further side reactions and consumption of Li ions. Once SEI layer forms, it allows Li ions to pass through into the electrode while keeping the electronic resistance high. There is an “energy window” associated with the electrolyte that should be in accordance with the chemical potentials (Fermi energy) of the anode and the cathode [9]. Fermi energies of anode and cathode materials should match the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the electrolyte in Li-ion batteries, respectively; otherwise the electrolyte will get reduced (anode) or oxidized (cathode). For instance, potentials (vs. Li−/Li) of graphite (0.05 V) and Si (0.2 V) are much lower than the HOMO energy of a common electrolyte of 1M LiPF6 in EC/DEC (1:1), which is about 1 V. In this case, an SEI layer is necessary to prevent the electrons passing from electrolyte to cathode and from anode to electrolyte [9]. It means that graphite owes its popularity mainly to the formation of SEI layer or else it would be useless in Li-ion batteries. Therefore, Si also needs a stable SEI layer formation in order to be used as an anode material. If the SEI layer breaks and gets pulled off the surface, new SEI layer forms. In this case, more Li ions are consumed, and more undesired residual compounds emerge as dendrites. If this occurs repeatedly, since these dendrites are not wiped out, it finally ends up with shortcut failure. A SEI layer should be stable and be able to repair itself quickly right after any crack emerges. However, silicon's high volume change during charging/discharging, widespread crack formation throughout the whole film, and therefore extensive exposure of fresh Si to electrolyte can further amplify the rate of SEI layer formation and quickly cause cell failure. Therefore, SEI formation on Si anode materials and its relation to film's microstructure and mechanical properties needs to be better understood. SEI layer formation on Si thin film anodes will be further discussed in the following sections.
Earlier Studies to Address Problems Associated with Si Anodes:
Previous studies have attempted several approaches to solve the problems associated with Si/Li intercalation summarized above. Coatings on Si, using active and inactive additives, Si—C composites, nanostructures of elemental Si and its composites, and Si thin films have been investigated for this purpose [1, 12, 18]. These methods were successful only to a certain extent and generally addressed few of the several issues of Si anodes. Using additives and coatings was considered to be an effective way to minimize the stress caused by high volume change. It also provides electronic contact between Si particles that considerably affects the capacity, which then provides enhanced capacity retention. On the other hand, theoretical specific capacity values of Si-based anode materials with active or inactive chemical additives typically range from 300-1700 mAh/g [12, 19], which is much lower than the about 3580 mAh/g capacity of elemental Si. Experimental values for these Si-based composite materials were found to be even poorer. For example, among the highest reported values, Si powder with carbon nanotube additives showed a capacity of 940 mAh/g [20].
Si nanowires were also tested as anode materials [21], and they have provided specific capacity values as high as 2800 mAh/g at the end of 10th cycle [5]. Nano-sized materials can significantly improve the lithium transport properties due to the high surface to volume ratio and high electrode porosity, and therefore, increase in the electrochemical properties has been expected [1]. Nevertheless, the capacity measurements were limited to only a few tens of cycles, which is not compatible for current commercial batteries. Amount of SEI layer formation is also increased with the enlarged electrode surface area, leading to more consumption of Li+, and thus decreasing the cycling performance. Recently, CNT-Si core-shell wire composites showed the specific capacity value of about 2900 mAh/g at the 80th cycle [22]. Cui et al. [23] reported a capacity of 1000 mAh/g and a capacity retention of 90% at the 100th cycle with nanowires that include crystalline Si cores and amorphous Si shells. Another study also reported CNT-Si—Ni nanowires achieving 2000 mAh/g specific capacity at the 100th cycle [24]. Several other Si—C composite approaches were also proposed to overcome the problems with Si anodes. Yi et al. [25] were able to produce Si—C composite materials showing specific capacity value of 1500 mAh/g at the 200th cycle. Focus of another study by Yu et al. [26] was to avoid the stress problems using Si ribbons patterned on soft substrates. A capacity retention of 84.6% was achieved at the 500th cycle with a capacity of 3498 mAh/g. Similarly, Wu et al. [27] proposed the development of Si nanotubes covered with a SiOx layer to improve the mechanical properties of Si anodes. Capacity values of 1200 mAh/g at the 600th cycle and 600 mAh/g at the 6000th cycle were achieved at 1C and 12C discharging rates, respectively, for these novel SiOx/Si nanotubes. To the best our knowledge, these results seem to be the highest specific capacity values over such large number of cycles ever reported among nanostructured Si anodes.
However, fabrication methods listed above are often complicated and high-cost, which make them difficult to be implemented in practical battery applications. Although Si—C composites produced by relatively more well-developed fabrication techniques such as pyrolysis, milling, and chemical vapor reactions have given reasonably good capacity retention (for instance, Kim et al. [28] showed that Si—C composite nanowires can provide a discharge capacity of 2768 mAh/g with the capacity retention of 87% at the 80th cycle), requirement of heat treatment, longtime of production, and in some cases need for nanostructured templates make those methods also significantly complex and expensive [12, 19].
On the other hand, thin film growth techniques such as sputter deposition or chemical vapor deposition are much more practical and inexpensive compared to the approaches listed above [30]. Thin film anodes have several advantages over bulk materials in Li-ion batteries. They usually provide better stability and capacity retention. They operate kinetically faster due to shorter pathways for Li-ion insertion/de-insertion. Si thin films have proven to demonstrate prominent results with extensively high specific capacities [12, 31]. However, for practical use of Si thin films in batteries with overall high capacities, they should be thick enough, at least in micron scales. Growing thick Si thin films is a major challenge since it generally suffers from stress build up during deposition followed by delamination from the substrate. Pulverization and stress problems become even worse when the thickness increases. Additional stress during lithiation/delithiation further decrease the mechanical stability. Although it is also a difficult task, improved adhesion of Si to the substrate can partially improve the resistance to stress and delamination [32]. Researchers tend to reduce the cycle number when the film thickness is higher because of the mechanical instability problems discussed above. To the best of our knowledge, considering the film thickness, cycling number, and the capacity values, the best performance was reported by Uehara et al. [31c] with a 1 μm thick Si thin film anode, which lasted for 200 cycles with a final specific capacity value of 1700 mAh/g. However, we note that high-capacity results in those studies were achieved only after additional processing steps such as substrate roughening by chemical etching to enhance film adhesion.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.